The use of blow molded thermoplastic containers and other hollow articles for a wide variety of liquids has gained wide spread commercial importance; see the article entitled "Blow Molding: The Next Five Years" in Plastics Technology, June 1979, pages 61-64. Blow molding permits the fabrication of lightweight, intricately shaped containers which are corrosion-resistant and mechanically strong. For many applications, such as the storage of aqueous or other highly polar liquids, these containers are, for all practical purposes, impervious to the contained substances. In other applications, however, where relatively nonpolar volatile organic fluids are to be held, blow molded containers have little if any value because of the ability of such volatile substances to diffuse through the walls of the thermoplastic containers. Nonpolar substances which are presently of greatest commercial interest as contained liquids include gasoline and other liquid fuels, hydrocarbon-based cleaning fluids and other household solvents, and oil-based paints containing volatile hydrocarbons. Diffusion of such fluids often results in an unacceptable loss of at least a part of the constituents making up the contained liquids. In the case of oil-based paints, the more volatile substances are lost by diffusion and the properties of the paint dramatically change to make it of little value.
Examples of thermoplastic materials or resins which have been employed in the production of blow molded articles include polymers and copolymers of styrene, acrylonitrile, vinyl chloride and olefins having at least one aliphatic mono-1-olefin with a maximum of 8 carbon atoms per molecule and PET (polyethylene terphthalate). The preferred types of thermoplastic materials for blow molding include polyolefins and copolymers of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-butene and 3,3-dimethyl-1-butene.
An additional problem with the containment of volatile nonpolar fluids in thermoplastic containers is the possibility that the concentration of flammable substances in the environment outside the container can reach a level to create an extremely explosive atmosphere.
In an attempt to overcome the direct diffusion of the volatiles through the walls of blow molded thermoplastic containers, a number of processes have been commercialized for treating their surfaces either during or after the blow molding process.
A post-treating method for providing a barrier coating and rendering the blow molded polyolefin bottles relatively impermeable to the passage of nonpolar solvents is described in U.S. Pat. No. 2,811,468. In this process, the surface of the previously formed blow molded bottle is fluorinated with pure fluorine or a mixture of fluorine and air or nitrogen. The resulting fluorinated containers have been found to be greatly improved in their barrier properties with respect to hydrocarbon solvents.
A more efficient and economical process for obtaining containers having improved barrier properties is described in U.S. Pat. No. 3,862,284, assigned to Air Products and Chemicals, Inc., the assignee of the present application, which process comprises blending 0.1 to 10% by volume fluorine and 99.9 to 90% by volume inert gas into a fluid medium prior to expanding the parison within the closed mold to conform the parison to the contour of the mold. The containers produced by this process, which has been designated as the AIROPAK Process, have an interior surface which possesses extremely high resistance to permeation by organic molecules; see the article entitled "Fluorination of Polyolefin Container, During Blow Moulding to Reduce Solvent Permeation" in Plastics and Rubber Processing, March 1979, pages 10-16.
Another commercially available process for improving the barrier properties of blow molded containers involves post-treating the container with a gaseous mixture of sulfur trioxide, ammonia and a dry diluent gas, which is known as the Dow sulfonation process; see the article entitled "Industrial Blow Molding: The Sleeping Giant Stirs" in Modern Plastics, November 1977, pages 34-37, at page 37.
The manufacture of barrier coated thermoplastic containers by any of the commercially available methods is hampered by the lack of a rapid, inexpensive quality control method to determine the effectiveness of the inner surface treatment in minimizing solvent loss by permeation through the walls of the container. One method is simply to directly measure the loss in weight of a container filled with the solvent over a period of time. However, this method requires days or even weeks in order to obtain a significant measurable loss of solvent through the walls of the container by direct diffusion. Obviously such a test is impractical for use as a production quality control method where it is essential to dectet any processing difficulties as soon as they occur so that immediate corrective action can be taken.
A number of quality control methods have been used to provide more rapid means for determining the impermeability of a container, such methods fall into two broad classes. The direct methods measure the permeability of the solvent through the walls of container. Indirect methods depend upon a measurement of characteristics other than permeability of the container, but which relate to permeability. The chief disadvantage of many of these prior art methods is that they result in the destruction of the container and a loss of production. Such losses can be very significant in the quality control of large blow molded containers such as 55 gallon high density polyethylene (HDPE) drums and HDPE gasoline tanks.
One direct method that is used is the pressure-accelerated permeability measurement method in which a sample is cut out of a container and mounted in a high pressure test cell. A liquid or gas is forced to diffuse under high pressure through the wall of the sample to the other side where its presence is detected either by chemical or physical means. In addition to the disadvantage of being one of the destructive quality control tests, it still may require hours or days to determine the permeability of a given sample.
Another direct method for permeability measurement is the dye test method comprising exposing the inner surface of the barrier-coated product, or a sample cut out of the product, to a solution containing an intensely colored or fluorescent dye, removing the solution after a given period of time, and examining by visual or instrumental means the degree and depth of dye penetration into the walls of the product. This method is not applicable to products incorporating dark colored and/or opaque pigments and tends to give erroneous results because the diffusion characteristics of low molecular weight, volatile substances may differ greatly from that of the complex organic dyes.
A number of indirect tests are available which include the chemical or physical detection of the active component in the barrier treatment such as fluorine or sulfur from the AIROPAK and Dow Processes, respectively. In addition, the measurement of optical or other physical properties such as contact angle or total reflectance is used. In those cases in which fluorine is used as the active barrier agent, X-ray fluoresence or combustion followed by chemical analysis are applicable. These methods often fail to detect containers with unsuitable barrier properties because a given surface may not be uniformly treated. The optical or physical property measurements have the disadvantage of being highly sensitive to contamination and are difficult to correlate with barrier properties. Finally, all such methods tend to be slow, tedious and relatively expensive to obtain permeation data on containers coming off a production line.